It is well known in the earthworking equipment industry that the useful life of a cutting edge or cutting bit of say, a ground engaging component, is increased if it has a combination of both wear and impact resistance. For example, equipment such as excavation teeth, excavation blades, mining plows, grading blades, impact blades and the like, which engage the ground, require both high wear resistance and fracture toughness. But it is also very crucial that the wear and impact component be bonded to the supporting base metal in a manner that the ground engaging tool does not fail due to a failure of the bond between the wear and impact component and the supporting base.
In the past, researchers at Caterpillar Inc., the assignee of the present invention, have developed composite materials having a combination of impact and wear resisting surfaces. One composite material includes a base member of austempered ductile iron and a plurality of hard particles such as tungsten carbide imbedded in the base member. The composite material may be prepared in a variety of ways. One way is to place the tungsten carbide particles into a mold and pour iron metal around them. The metal is solidified by cooling and then austempered. Another way is to place hard inserts made from a hard paste of tungsten carbide on the surfaces of a polystyrene foam pattern. The foam pattern is placed in a sand mold and during casting, the iron replaces the polystyrene and infiltrates the hard particle paste. The iron is solidified and then the composite is austempered.
Other methods developed at Caterpillar Inc. include techniques where abrasion resistant materials are welded on a surface or into cavities in the metal base comprising the ground engaging tool. Although the foregoing techniques have been very successful, there is a desire to continuously improve the wear and impact resistance of such components used for making ground engaging tools in order to enhance quality and maintain a competitive edge in the global marketplace.
There has been a long-felt need for having cast-in-place metal matrix composites that have a combination of abrasion resistance, impact resistance and a high strength bond between the metal matrix surface and the metal base surface. The wear resistance is achieved by increased hardness of the metal matrix composite while high impact strength is attained by increasing the fracture toughness of the, metal matrix composite. However, the bond strength at the metal matrix-base metal interface is typically not high enough for the composite to perform very well in a rigorous environment to which a ground engaging tool is exposed.
Several pressure infiltration casting processes are well known in the industry. U.S. Pat. No. 5,004,034 issued to Park et al. discloses a process for forming a metal matrix composite between at least two bodies having similar or a different chemical composition. The metal matrix composite is produced by a spontaneous infiltration technique by providing a preform with an infiltration enhancer or infiltration atmosphere, which are in communication with the preform at least some point during the process. Molten infiltrating metal or matrix metal then spontaneously infiltrates the preform, whereby the metal matrix composite serves to bond together two or more bodies.
U.S. Pat. No. 5,188,164 issued to Kantner et al. discloses a process for forming a metal matrix composite by the application of a self-generated vacuum infiltration without the application of any external pressure or vacuum.
U.S. Pat. No. 5,322,109 issued to Cornie, and incorporated herein by reference, discloses a method for pressure infiltration casting wherein the steps of preheating and evacuating a mold cavity and infiltrant charge are carried out in a separate vessel from the pressure vessel wherein the mold cavity is filled using a vent tube. This process evidently allows for rapid finished article throughput.
It has been desirable to have an improved pressure infiltration casting process for making articles that have cast-in-place metal matrix composite components that exhibit a combination of wear and impact resistance properties and a high bond strength between the metal matrix and the supporting base metal.
The present invention is directed to overcome one or more problems of heretofore utilized pressure infiltration casting processes used for making articles having cast-in-place metal matrix composites.